One possible field of use of the invention is the catalytic steam reforming of hydrocarbons for the production of a hydrogen-rich gas. As process gas a mixture of steam, hydrocarbon and/or carbon dioxide is used. This gaseous mixture is fed at temperatures of about 400.degree.-600.degree. C. and pressures up to 4 MPa to a reaction chamber which may consist of a plurality of catalyst-filled tubes and is heated there to about 750.degree. to 800.degree. C. This gaseous mixture then reacts endothermically via the catalyst to form a hydrogen-rich gas containing portions of carbon monoxide, carbon dioxide and excess steam and hydrocarbon.
The hydrogen content of the product gas produced depends, among other things, on the excess of steam used as well as on the temperature and pressure in the catalyst-filled tube. An increase in the excess of steam and in the temperature within the catalyst-filled tube result in an increase in the recovery of hydrogen, while an increase in pressure results in a decrease thereof.
From EP 0 194 067 B1, an apparatus for the carrying out of an endothermic catalytic reaction of a stream of process gas is known. The disclosed apparatus has a housing with one or more catalyst-filled tubes arranged parallel to each other therein, each having a blind end. Within the housing, means are provided for the feeding and discharge of a flue gas for heating the outer surface of the catalyst-filled tubes through the inside of which the stream of process gas is conducted, thereby absorbing heat. The catalyst-filled tubes are in each case surrounded by a tubular shell which forms an annular space with the tube. The shell extends over the greater part of the catalyst-filled tube. The feed for the flue gas is directed directly toward the blind end of the catalyst-filled tube. The direct flow effected thereby of the hot flue gas against the catalyst-filled tube proves disadvantageous since in this way the thermal load on the blind end of the catalyst-filled tube is very high. At critical points, such high temperatures are produced in the shell of the catalyst-filled tube that it becomes damaged within a relatively short period of time and must be replaced unless a suitable cooling of the flue gas has been previously provided. This could be done, for instance, by a preliminary heat exchanger or by mixing with flue gas which has already been cooled. Both of these measures require corresponding additional structural expense.
In order to mitigate the thermal load on the catalyst-filled tubes, it is known to provide them on their outside with a ceramic protective covering in the region of the direct action of the hot flue gas. One disadvantage of such protective coverings lies in their susceptibility to thermal expansion stresses, which leads to damage to the ceramic and then, merely with a time delay, also to damage to the catalyst-filled tubes themselves.
In the non-previously published Patent Application EP 89 250 073.7 of the present assignee, it is proposed, to solve the problem of overheating of the walls of a reaction chamber, to conduct the hot flue gas used for the heating of the walls of the reaction chamber, first of all, along a barrier consisting of a temperature-resistant material which is a good conductor of heat, thereby allowing it to give heat off to said barrier until it has reached a temperature which is harmless for the walls of the reaction chamber. Only then is the stream of flue gas deflected and conducted further in the opposite direction on the other side of the barrier, the flue gas which has already cooled down somewhat coming into direct contact with the walls of the reaction chamber and giving heat off to these walls. Since the flue gas, however, at the same time, takes up heat from the heated barrier, its temperature is maintained practically constant despite the heat given off to the reaction chamber in the region of the barrier, which extends only over a part of the length of the reaction chamber. Outside the region of the barrier the flue gas cools relatively rapidly down on its path along the reaction chamber so that the temperature within the reaction chamber shows a correspondent gradient.
While in EP 89 250 073.7, only thermal damage to the walls of the reaction chamber is to be avoided, the object of the present invention is to optimize the thermal conditions for an endothermic reaction, particularly a catalytic reaction, so that substantially isothermal conditions prevail within the entire reaction chamber.
This means that the reaction chamber will no longer, as previously customary, have a strong temperature gradient with which the desired endothermic reaction takes place with correspondingly strongly different intensity in the different regions, but that a better utilization of the reaction chamber is obtained, in the manner that approximately equally good reaction conditions from a thermal standpoint are created practically everywhere.